Five to 10% of the adult population will develop a clinically apparently thyroid nodule in their lifetime. Only a small proportion of these individuals, however, will turn out to harbor a malignant neoplasm. Differential diagnosis of thyroid nodules is often problematic prior to surgical resection, and even after careful histological examination of the resected tissue the nature of some follicular neoplasms remains unclear. The experiments outlined in this proposed aim to identify molecular factors associated with abnormal thyroid cell growth, in an attempt to increase our understanding of the biology of these tumors. In the first portion of these studies, we will determine the clonal characteristics of the various types of thyroid adenomas and carcinomas. This will be done by X-chromosome in activation analysis, taking advantage of polymorphisms in the X-chromosome genes hypoxanthine phosphorybosiltransferase (HPRT) and phosphoglycerate kinase (PGK). Evidence pointing to a monoclonal origin of these tumors will provide support for a somatic mutation model of thyroid neoplastic formation. Clonal studies will also be performed on multiple nodules obtained from patient with multinodular goiters, to assess the frequency of monoclonal neoplasms following exposure to goitrogenic stimuli in these patients. In the second part of the proposal we will examine thyroid tumor DNA for structural mutations of proto-oncogene sequences. These experiments have been prompted by our preliminary data showing H-ras gene rearrangements in some thyroid neoplasms. Polymerase chain reaction DNA amplification of thyroid tumor DNA will also be performed to screen for the presence of ras gene point mutations.